Aircraft velocity-altitude meter



United States Patent AIRCRAFT VELOCITY-ALTITUDE NIETER Frank G. Willey, East Hills, Roslyn Heights, and Francis D. Greenleaf, Syosset, N.Y., assignors to Servo Corporation of America, New Hyde Park, N.Y., a corporation of New York Application December 13, 1956, Serial No. 628,172

Claims. (Cl. 25083.3)

Our invention relates to an improved correlation computcr and has particular application to an airborne scanner including automatic means for determining the velocity-altitude function (or v./h. rate) of an aircraft.

It is an object of the invention to provide an improved device of the character indicated.

It is also an object to provide a simplified and improved correlation computer.

Another object is to provide an improved correlation computer which is inherently polarity-sensitive, that is, a computer which can yield an output of one sign (polarity) reflecting a first directional deviation (between two closely related signal functions) away from maximum correlation, and yielding an output of opposite sign (polarity) reflecting a second deviation (between the same signal functions) in the opposite direction away from maximum correlation.

It is another object to provide an improved automatic means for continuously determining the velocity-altitude function of an aircraft.

It is a further object to meet the above objects with a device which is completely passive (i.e., essentially non-radiating) Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, preferred forms of the invention:

Fig. 1 is, a simplified electrical diagram schematically showing significant components of an automatic correlation device of our invention;

. Fig. 2 is a graphical representation of a correlation function and of a related function developed by our invention; and

Fig.3 is a diagram similar to Fig. 1 but illustrating application of the invention to an airborne velocity-altitude meter. 1 Briefly stated, our invention contemplates an improved correlation computer for continuously evaluating the extent of correlation between two closely similar functions of the same variable. In the forms to be described, the two similar functions vary with respect to time and differ from each other by an unknown time displacement which is to .be ascertained by the automatic mechanism of our invention. We introduce an adjustably variable delay in that function which leads the other function, so that there may then be said to be two closely synchronous similar functions. We then differentiate one of said closely synchronous functions (to the exclusion of the other) with respect to time, and multiply the undifferentiated function by the dilferentiated function, thus developing an output signal exhibitinga null (i.e., passing through zero) when the adjusted delay produces maximum correlation, and oppositely polarized for adjusted delays shorter or longer than necessary to produce maximum correlation. Thus, the instantaneous magnitude and polarity of the product, curve can be utilized for corrective adjustment 1 two closely similar functions, an early function S (t'r),

and a later function S 0), available respectively in the inputs 8-9; the signals at inputs 8--9 may be electrical video signals in separate lines 8-9. The leading function is subjected to an adjustably variable delay b at delay means 10 so that, when one compares the output of the delay means 10 with the undelayed function, there result two closely synchronous similar functions of the same variable (time). In accordance with our invention, we subject one of these closely synchronous functions to differentiation with respect to the variable (time), and in the form shown, it is the undelayed signal which is fed to differentiating means 11. We then multiply, at 12, the differentiated signal by the undifferentiated signal, and thereby develop the following product:

The integral of this product passes through zero at the point corresponding to maximum correlation (i.e., when 1-=b), being positive one sided this point (i.e. for one polarity of difference between 7' and b), and negative on the other side (i.e. for the opposite polarity of difference between '1' and b). Thus, when multiplied closely synchronous signals involve too little delay to develop maximum correlation, a product of first polarity develops at the output of multiple 12; and when the delay is too great to develop maximum correlation, a product of opposite polarity develops. This situation is graphically depicted in Fig. 2, wherein the solid curve represents the correlation function of the integrated product of two undifferentiated functions, maximum correlation being identified with a maximum amplitude x, when the inserted delay d equals the lead time 1- between the two input functions. Fig. 2 also illustrates, at the dashed curve, the integrated product of the differentiated and undifferentiated functions, said product passing through zero at 1):? and exhibiting oppositely polarized values for products involving undercorrected and overcorrected delay at 10.

In accordance with a feature of the invention, the directionally polarized output of the multiplier 12 is utilized to effect corrective adjustment of the delay inserted at 10, in the direction tending to produce continuous tracking for maximum correlation between the two signals. In the form shown, this adjustment is made by mechanical means driven by the shaft 15 of motor means 13. The amplifier 14 suggests processing of the signal appropriate for driving motor 13, and since the position of the motor shaft 15 represents the adjusted delay b for which correlation is achieved, this shaft position (and, therefore, the unknown time displacement -r) may be directly observed, as by noting the position of an indicator arm 16 against a scale; alternatively, the arm 16 may be part of a potentiometer 17, so that electrical output at terminals 18 may directly reflect the unknown time displacement 1'. In order that the delay adjusting mechanism may function in response to the time integral of the product developed at 12, it is preferred that the overall response of elements 13-14 shall be characterized by a time constant substantially longer than the shortest periodical component of S (t), said time constant being nevertheless shorter than the shortest periodical component reflecting variation in the time displacement 1. d

In the arrangement of Fig. 3, we show application of Patented Dec. 22, 1959 Such the shaded portions. Such detectors are responsive to .in-

frared radiations, andby means of infrared transmitting optics, such as.the.lensl22 ,.the sensitiveareas ofcells 20-121 maybe imageduppn the ground, preferably directly, beneath the aircraft. Also preferably, the cells 29-521. and the-mounting platform 23Ato whichthey are secured, and the.lens' 2 2, areall held in a CQIltiIll-lr ously stabilized. position, with the optical. axis 23 oriented vertically and with the platform 23A orienting both cells. 20,-21 in the same downwardly facing horizontal plane. Such stabilizing meehanisrns are well known and are, therefore, notnecessary for the present description.

It sufiices to say that the optics 22 continuously image the sensitive areas of cells 20-21 on the ground, so that with the forward motion of the aircraft, each of the sensitive areas of these cells traces substantially thezsarne scanned path on the ground. This fact is suggested by the legendconnoting image motion in thedrawing; Be: cause of the physicalspacing between the sensitiveareas of cells,20--21 and because they both scan substantially thesame path on. the ground, the image at the ground of one of them geometrically leading that of the other, they will each develop in their respective output lines 24-:25, video signals which have substantially the same function ofitime, the only difference being that the video output of the lead cell 20 leads the video output-ofrthe trailing cell 21 by an amount proportional tothevelocity altitude function (the v./h. rate) in the aircraft.

The two outputs 2425 .may be fed to the two input-signal connections 8-9 of the circuit of Fig. 1, and the v./h. rate may insuch case be determinedmerel-y by observing the output at 18. However, in the form shown in Fig. 2, we utilize'the inherent differentiatingsproperty of.the magnetic recording and reproducing function to achieve both adjustably variable delay and differentiation in the same piece of equipment. Thus, the outputuof the trailing cell 20, when suitably amplified at 26, may be fed directly to the multiplier 12,..and the output ofpthe lead cell 21 may be amplified; as necessary, at'27' for application to the recording head 28 of acontinuously recycling single-channel magnetic-recording dr'um. 29, drivenby motormeans. 30. Inthe formshown, adjustably variable delay is achievedrbymountingthe pick-11p head 31 on an arm 32 'adjustably positionable about the rotating axis. 33 ofthe recorder drum .29. An" erasing head.;3.4 electrically. connected to biasing means 35;.cona

tinuously reconditions the channel 29, for variablecdelay" offresh video, as recorded at 28;.

The outpntof the pick-upgheadfil will reflect differentiation of the video developedbythetlead cell [21":and itgwill also reflect delay representedby the travel time forthe recording materialbetween: the. heads'128.I-\31. This output may be conditioned by an amplifier 36 for supply to the multiplier 12. 1 r

In the form shown, the polarized output of th'eymultiplier 12 is conditioned by amplifiermeans'3i7vto modu-. late (at 38) one of. the output phases of ar'split-phase source 39; themodulated signal excites thetcontrolrwinde. i g, 4Q of a directionally controllable two=phase-motor 41 The other output phase of: the. source 39.:is190.

displaced from the first .out-putphase' and excites the. refs erencewinding 42 of motor 41. The shaft output .OffIIIOJ-i s; t s anqstqdl avimeassfi to. remsition the arm 32, as necessary, to produce maximum correlation. At equilibrium, the shaft position at 43 will reflect the velocity-altitude function and, therefore, potentiometer 44 may develop a voltage proportional to displacement. Actually, the greater the delay, the lower the v./h. rate, so that the output of potentiometer 44 is an inverse function of v./h., meaning proportional to h./v., as noted in Fig. 3. To get a direct reading. of v./h'., therefore, we show an inverter. 45 supplying. a v./h. indicator 46.

It will be appreciated that we have described. a basically simple mechanism for continuously tracking thedelay between two similar functions of time, regardless of how this delay may vary, as a function of time. Substantial simplification over previous methods of performing this function is achieved by electrical differentiation prior to correlation, so that the differentiating function which is necessary in evaluating a correlation is automatically performed prior to correlation. The correlation function thus is .inherentlysuch as to provide a polarizedsensing signal for corrective adjustment of thedelay, as neces: sary to permitcontinuous tracking.

While we have described the inventionin detail for the preferred forms shown, it will be understood that modifications may bemade within the scope of the invention as defined in the claims which follow.

We claim;

1. Angaircraft velocity-altitude meter, comprisingtwo energy-responsive elements, optical means for imaging said elements on theground' in anorientation such that due to the motion of the aircraft the path scanned by one of said elements coincides substantially with the path scanned by the other ofsaid elements, said elements being physically spaced ifrorneach other such :that: 1 the. image of one of said elements geometrically leads that cof the other on the ground, whereby saidelements develop closely similar video signals which essentially differ from each other by an unknown time delay reflecting the instantaneous image motion or velocity-altitude function, means for adjustably delaying the video output of the element developing the lead image with respect to that of the other element, whereby there may be said to be available two closely synchronous similar functions of time, means for differentiating one of said functionswith respect to time to the exclusion of the other of said functions, means formultiplying the resultant differential by the other of said functions, and polarity-sensitivecontrol means responding to the output of said multiplying means and connected in corrective adjustably varying relation with said delay means, whereby the adjusted delaywill continuously reflect tracking at maximum correlation and will reflect the instantaneous relation'between velocity and altitude of the aircraft.

2. A device according to claim- 1, and including an indicator responsive tothe instantaneous corrected delay setting of said delay means.

3. An aircraft velocity-altitude meter, comprising two energy-responsive elementsgoptical means. for imaging said'elements on the groundin an orientation such'that due to the motion of 'theaircraft the path scanned by one of said elements coincides substantially with the path scanned by the other of said elements, said elements being physically spacedfrom each other such that the image of one of said elements geometrically leads that .ofthe other on the ground, whereby said. elements develop closely similar video signals which essentially differ from each other by.an unknown time delay reflecting. the. in-. stantaneous image motion, or velocity-altitude function, means for adjustably delaying the video output of .the element developing the lead, image with. respect to. that of theother element, said last. defined means comprising: a magnetic recording device with, recording, andpickz-up heads; whe reby inherent inthe operation of said. record-. ing device the: signal applied thereto. is: extracted therefrom; as substantiallyv the derivative of: said signal 11 with respect to time, means for multiplying the output of said recorder by the video output of the cell having the trail- 7 ing image on the ground, and polarity-sensitive means responsive to the output of said multiplying means and connected in 'adjustably varying controlling relation with the delay between recording and pick-up at said recording means, whereby the adjusted delay will continuously reflect the instantaneous relation between velocity and altitude of the aircraft.

4, A device accordingto claim 3, in which said recording means comprises a continuously rotated drum with a continuous band of magnetic-recording material, a recording head and a pick-up head, one of said heads References Cited in the file of this patent UNITED STATES PATENTS 10 2,166,991 Guanella July 25, 1939 2,688,124 Doty et al Aug. 31, 1954 2,718,638 De Rosa et al. Sept. 20, 1955 2,800,654 De Rosa July 23, 1957 

